Particle accelerators, including synchrotrons, make use of various coatings on interior surfaces of the accelerators. For example, three types of coatings generally used on the interior surfaces of a particle accelerator are: (1) superconducting coatings; (2) coatings that have low coefficient of secondary electron emission; and (3) getter coatings, specifically non-evaporative getter (NEG) coatings. A getter is a deposit of a material that is placed inside a vacuum system for the purpose of decreasing the pressure in the vacuum system and maintaining a vacuum. Getters can be used in a vacuum system to reduce the pressure to less than 7.5×10−10 torr in the system.
Most of these coating can be deposited on surfaces by sputter deposition or sputtering, a physical vapor deposition (PVD) technique. In sputter deposition, particles are ejected from a solid target material due to bombardment of the target by energetic particles (e.g., gas ions). For example, a NEG coating can be generated by sputtering a compound wire or a twisted wire system (e.g., often titanium (Ti), vanadium (V), and/or zirconium (Zr)) to produce a metal alloy coating.
The technique of wire sputtering (i.e., sputtering a wire to deposit a material on interior surfaces of a tube or other structures) has issues, however, including: (1) a wire needs to be thin and under tension (e.g., to address thermal expansion) to coat the interior surfaces of small diameter tubes, and it tends to fracture; (2) the wire needs to be centered on-axis everywhere, which is difficult for very large length/diameter ratio tubes; and (3) the sputtering process gas needs to be static or near-static as pressure gradients produce different thicknesses (i.e., non-uniformities) in the deposited material.
Alternatively, atomic layer deposition, a layer-by-layer chemical vapor deposition (CVD) technique, or pulsed laser deposition (i.e., laser ablation), may be used to deposit some materials. Pulsed laser deposition (PLD) is a PVD technique in which a high-power pulsed laser beam is focused on a target of the material that is to be deposited. Material is vaporized from the target in a plasma plume and is deposited as a thin layer on a surface. Pulsed laser deposition shows great flexibility since practically all materials can be ablated when laser light of sufficient intensity is focused on a surface of a target. The needed high laser intensity (i.e., power density) can be achieved with pulsed lasers and depends on the laser pulse duration, laser wavelength, and the composition of the target material. The deposition of laser-ablated material using pulsed laser deposition is commonly performed on flat or structured substrates that are placed near the focal point of the laser. In some instances, substrates can be placed off-axis or in an eclipse configuration (i.e., with a shield between focal point and substrate) to modify the microstructure of the coating.